Cranes are used to raise, shift and lower heavy objects or loads by means of a projecting, swinging arm or boom in conjunction with a hoisting apparatus. The hoisting apparatus can comprise a cable extending adjacent the boom, wound around a sheave situated near the inner end of the boom at one end, and having a load block assembly attached at the other end. The load block assembly can consist of an upper load block or boom point sheave assembly attached at the outer end of the boom or boom point and a lower load block or hook assembly, which includes a movable sheave and a load attachment means. The cable extends past the upper load block to support the lower load block. During the normal course of operation of the crane, the lower load block is maintained at a distance below the upper load block.
Such a crane can contain various moving parts which are coordinated to facilitate the attachment and transportation of objects. The boom can have the capacity for being elevated and lowered, as well as having extendable and retractable telescoping boom sections. Additionally, the cable can be reeled in and let out over the inner boom sheave. The retrieval and transportation of a load is achieved through various combinations of reeling in and paying out the cable, raising and lowering the boom, and extending and retracting the telescoping boom sections.
There is a danger, during the operation of the crane, of the lower load block coming in contact with the boom point, upper load block or other components attached thereon. Such contact could result in costly damage to the crane or its load. The cable could break resulting in the load and lower load block falling to the ground. The load could detach from the lower load block and fall. Alternatively, the load block could move over the boom point with the resulting interference damaging the components at their point of contact. Each possibility is clearly undesirable.
"Two-blocking" refers to the condition when the lower load block comes in contact with the upper load block. Extending or lowering the boom without sufficiently unwinding the cable and reeling in the cable too far can cause two blocking. As mentioned previously, it is important that the lower load block be prevented from coming with the upper load block. Therefore, cranes installed with a load block assembly would benefit greatly from an anti-two block device.
There are safety control systems adapted for installation on extension booms that prevent two-blocking. In a typical system, a switch is installed near the upper load block. The switch has a lever with a chain and a weight attached thereon. The cable is threaded through the weight. The weight maintains the switch in the working position. As the lower load block comes in contact with the weight, the tension in the chain relaxes, thereby releasing the lever of the switch which stops all activity which could endanger the boom apparatus.
There are problems with the prior safety control devices such as the one just described. The weight continually exerting a force on the switch causes a stress on the switch and shortens its life span, thus reducing the reliability of the device.
The present invention overcomes one or more of the foregoing problems and achieves one or more of the aforementioned objectives.